With the reduction in dimensions and the increase in frequency and integration density of MOS transistors, power amplifiers constructed using MOS transistors are rapidly increasing. In radio systems and the like in which a reduction in power consumption is required, transmitters with class C amplifiers having excellent power efficiency are frequently used.
Class C amplifiers have the problem that the power efficiency and output power are influenced by threshold shifts of MOS transistors. Moreover, there is the problem that a shift in characteristics of the MOS transistors causes a shift in operating states of transistors in an output stage to deteriorate the reliability of a transmitter.